MR imaging is widely used today as it allows the recording of two-dimensional or three-dimensional image data which can portray the structures in the interior of an examination object with high resolution. During the MR imaging, the nuclear spins of hydrogen nuclei in the examination object are aligned in a main magnetic field (B0) and then excited as a result of irradiation by HF (high-frequency) pulses. The excited magnetization is detected, wherein spatial encoding can be achieved using various known methods.
The time that is required for data capture can be reduced by means of parallel data capture using a plurality of receive coils. However, such a data capture can cause a degradation of the signal/noise ratio. For example, it is not possible to separate signal components of different voxels in the k-space data that is captured by a plurality of receive channels. The resulting degradation of the signal/noise ratio is frequently quantified by a location-dependent geometry factor (g factor).
In addition to parallel data capture using a plurality of receive coils, parallel excitation (“parallel transmit”) using a plurality of transmit channels is also possible in MR imaging. The plurality of transmit channels can be a plurality of transmit coils, each of which can be so controlled (in a monitored manner) that a desired locally varying excitation profile is produced. The parallel excitation and parallel data recording can be performed using the same coils or coil segments that are coupled respectively via a transmit/receive filter to both a transmit path for excitation and a receive path for the data recording.
Examples of MR imaging which uses a plurality of transmit channels for excitation and a plurality of receive channels for data recording are described in Lawrence L. Wald, Elfar Adalsteinsson: “Parallel Transmit Technology for High Field MRI”, MAGNETOM Flash 1/2009, pages 124-135 (2009), Siemens AG, Erlangen, Germany and in Andrew G. Web, Christopher M. Collins, “Parallel Transmit and Receive Technology in High Field Magnetic Resonance Neuroimaging”, International Journal of Imaging Systems and Technology—Special Issue on Neuroimaging, Vol. 20, 2-13 (2010), Wiley, New York, USA.
The parallel excitation, for generating an excitation profile that can be modified in more than one dimension, can be used for the purpose of at least partially reducing the degradation of the signal/noise ratio that occurs when data capture is effected using a plurality of receive channels. By limiting the excitation to the “region of interest” (RoI) of the examination object, for example, it would be possible to reduce signal degradation caused by correlated noise of voxels outside of the RoI. In order to achieve such a localization of the excitation, it can however be necessary to use long excitation pulses and/or a high excitation power. This is undesirable.